drm/i915: Move intel_lrc_context_pin() to avoid the forward declaration

Just a simple move to avoid a forward declaration, though the diff likes
to present itself as a move of intel_logical_ring_alloc_request_extras()
in the opposite direction.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161218153724.8439-2-chris@chris-wilson.co.uk